Modern drilling techniques employ an increasing number of sensors in downhole tools to determine downhole conditions and parameters such as pressure, spatial orientation, temperature, gamma ray count etc. that are encountered during drilling. These sensors are usually employed in logging while drilling (LWD) and ‘measurement while drilling’ (MWD). The data from such sensors are either transferred to a telemetry device, and thence up-hole to the surface, or are recorded in a memory device by logging tools to be used in a later time.
One type of telemetry method is electromagnetic (EM) telemetry, which uses a downhole EM transmitter to create very low frequency EM carrier waves in the formation adjacent to the well that are detected at the surface. In EM telemetry systems, the downhole carrier signal is produced by applying an alternating electric current across an electrically isolated (nonconductive) portion of the drill string. The required isolation is provided by a mechanically strong gap in a portion of drill string (called a ‘gap sub’) in order to maintain the torsional, bending etc. properties required for the drilling process. The EM signal originating across the gap is subsequently detected on the surface by, in general, measuring the induced electric potential difference between the drill rig and a grounding rod located in the earth some distance away.
Nonconductive materials forming the isolation section of the gap sub typically have inherently less strength and ductility than the conductive steel materials of the drill pipe, giving rise to complex designs that are necessary to complement the structural strength of gap within the drill pipe.
As described by several patent publications, many types of electrical isolation arrangements exist for the purpose of signal transmission in a drill string. Although these systems electrically isolate and seal while being subjected to drilling loads, they generally do so with a complicated multi-component design that thus becomes a relatively expensive device. Examples of such complicated and expensive designs are disclosed in U.S. Pat. Nos. 6,158,532 and 6,050,353 assigned to Ryan Energy Technologies, Inc. (Calgary, Calif.) whereby many separate components of the assembly are shown to be necessary in order to resist axial, bending and torsion forces.
It is also common knowledge in the oil and gas industry that a two-part epoxy-filled gap between coarse threads can be used to resist both axial and bending loads. Reverse torsion, which would tend to uncouple the joint, can be resisted by the insertion of dielectric pins into carefully fashioned slots. Since epoxy does not adequately seal against drilling pressures of typically 20,000 psi, additional components must be included to provide an elastomeric seal, again leading to mechanical complexity and added cost.